fd: Let readv_nonblock() recover from EINTR.

fd: Let readv_nonblock() recover from EINTR.

No need to fail the operation if the read was interrupted by a signal.

The patch also fixes some minor issues in the documentation:
a typo ("The" was incorrectly capitalized) and a missing \ref
for xwrite().  We use the opportunity to get rid of the \a and \p
font annotations. This improves the readability of the source code,
which is more imporatant than nice looking web pages. However, we
only touch those parts of the documentation which are modified anyway.